If your New Year's resolve to become more physically active has started to flag, the findings of several studies may help renew your commitment. Research has already documented that higher levels of physical activity can help prevent or ameliorate many conditions that reduce function and hamper independence as we get older, including heart disease, stroke, cancer, diabetes, osteoporosis, and depression. Various types of exercise have also been linked with a reduced risk for dementia and Alzheimer's disease.

Now four studies, including two randomized trials, add further evidence that regular exercise may be the best thing we can do to stay not only physically healthy but also cognitively sharp into old age. One trial even suggests that exercise can help reverse early cognitive impairment in older women.

Three of the studies appeared in the Jan. 25, 2010, issue of Archives of Internal Medicine. In the first study, Harvard researchers analyzed health data from more than 13,000 women participating in the long-running Nurses' Health Study. They found that the women who reported getting the most exercise at age 60 were almost twice as likely to become successful survivors, compared with the most sedentary women. (A "successful survivor" was defined as living beyond age 70 without developing cognitive, physical, or mental health limitations or any of 10 major chronic conditions, including cancer and cardiovascular disease.) Successful survival was associated with a level of exercise equivalent to walking briskly five to six hours per week.

In the second study, Canadian researchers randomly assigned 155 women ages 65 to 75 to a once- or twice-weekly progressive strength-training program that used free weights and weight machines -- or to a control group that practiced toning and balance exercises twice a week. After one year, the women who did strength training improved their performance on tests of executive function -- the higher-order thought processes required for organizing, planning, and making choices. In this study, tests showed improvements specifically in the areas of selective attention and conflict resolution. These in turn were associated with improved walking speed (a predictor of falls and fracture risk). The control group didn't improve in these areas.

Previous studies have identified links between aerobic exercise and improvements in executive function in older people. This cognitive benefit was thought to derive from improvements in blood flow to the brain. It's unclear how strength training (which isn't aerobic) confers the same benefit.

In another study of exercise and cognitive function, scientists tracked 3,903 Bavarian women and men ages 55 and over for two years. They found that participants who reported exercising at a moderate level (less than three times a week) or high level (three or more times a week) at the start of the study were half as likely to have developed dementia two years later, compared with those who got no exercise.

A fourth investigation, which appeared in the January 2010 Archives of Neurology, examined the effects of vigorous aerobic exercise on mild cognitive impairment (MCI) -- a transition stage between normal age-related cognitive changes and dementia. People with MCI are five to 10 times more likely to develop dementia than people with normal cognitive function. Seattle-based researchers randomly assigned 33 women and men with MCI (average age 70) to either high-intensity aerobic exercise -- 45 to 60 minutes a day, four days a week -- or a stretching activity control group. Physiological and cognitive tests were administered before, during, and after the six-month trial. Overall, the aerobic exercisers had greater improvements in cognitive functioning than the control group did, with the women showing improvement on all measures of executive function, while the men improved on only one test.

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